Greetings all! I recently had a running email conversation with the "Grower Services & Product Development Director" at ProMix (aka Premier Tech). I began the conversation by posting a question on their website, asking if I should be treating ProMix HP as soil or soil-less when mixing...
www.420magazine.com
If you want to continue to pH the nute solution, I'd go with 6.3-6.7.
If your soil is buffered, the manufacturer of your nutes does not matter. MegaCrop is not special in that way. It's just chelated differently from most other nutes, which has no bearing on the pH of your substrate.